In the natural-circulation boiling water reactor (referred hereunder to as the natural-circulation BWR), in order to secure a natural circulation flow rate, a pressure vessel of the reactor is arranged with an axial long length and a reactor core is arranged at a relatively lower position within the pressure vessel of the reactor so as to form a large free space called a chimney over the reactor core.
The natural-circulation BWR does not include a re-circulation pump inside the reactor (internal pump) and a reactor re-circulation system (including a re-circulation pump outside the reactor and a jet pump) unlike a forced-circulation boiling water reactor (BWR), so that the fluid within a reactor pressure vessel is not to be forced-circulated by the recirculation pump inside the reactor.
In the natural-circulation BWR, the natural circulation flow rate is determined in accordance with the balance of a density difference between a downcomer part and the reactor core, that is, the pressure difference between vapor/liquid two-phase flow in the reactor core, and liquid flow in the downcomer. This natural circulation flow rate is ensured by increasing the water-level (head) of the downcomer part by elevating the reactor pressure vessel as well as by forming a chimney, which is a large free space, above the reactor core so as to reduce the pressure drop of the vapor/liquid two-phase flow in the chimney for reducing the water-head and to increase the water-head difference (head difference) due to the density difference between the inside and the outside of the shroud.
The chimney formed above the reactor core of a large-scale natural-circulation BWR is a very large free space with a radius of about 5 m and a height of about 10 m (see Patent Document 1: Japanese Unexamined Patent Application Publication No. HEI 02-80998, for example). When the free space is formed above the reactor core and the vapor/liquid two-phase flow discharged from the reactor core passes therethrough, a multi-dimensional flow is generated in the large space (chimney), which may prevent the natural re-circulation flow, providing a problem in the development of the natural-circulation BWR. The phenomenon of the multi-dimensional flow has been confirmed in the Russian natural-circulation BWR Vk-50.
Further, in order to figure out the behavior of the thermal flow in the chimney formed above the reactor core, a test of the vapor/liquid two-phase flow within the so-called large caliber vertical piping was performed in Ontario Hydro Technologies Canada.
This vapor/liquid two-phase flow test is a high-temperature and -pressure test using the vertical piping with a diameter of about 60 cm. From this test, it has been understood that the flow within the vertical piping with a diameter of about 60 cm is not a multi-dimensional but a one-dimensional stable flow.
On the basis of the result from the Canadian vapor/liquid two-phase flow behavior test, in a large-scale natural-circulation boiling water reactor, such as an SBWR, for ensuring the vapor/liquid two-phase stable flow in a chimney region, which is a large free space, a rectangular-columnar divided chimney composed of a plurality of square lattices is adopted. The divided chimney is about 60 cm square in size, and in the rectangular-columnar divided chimney with this size, the vapor/liquid two-phase stable flow is ensured like in the test in Ontario Hydro Technologies Canada.
In the natural-circulation BWR, by adopting the divided chimneys, the flow in each divided chimney is not the multi-dimensional flow, but it becomes a one-dimensional stable flow, enabling the stable natural circulation flow rate to be secured.
A natural-circulation reactor adopting the divided chimneys includes the technique disclosed in Patent Document 2 (Japanese Unexamined Patent Application Publication No. H04-259894). This natural-circulation reactor ensures preferable natural-circulation characteristics as well as suppresses the transient reduction in water level by adopting the divided chimneys. In the natural-circulation reactor adopting the divided chimneys, the chimney is vertically divided into two sections so that the flow-path sectional area of the upper divided chimney is smaller than that of the lower divided-chimney.
By adopting the divided chimneys in that a divided-chimney region is vertically divided into two sections so as to make flow-path sectional areas different from each other, the stable natural circulation flow rate can be secured while the stability may deteriorate. In general, the stability of the natural-circulation BWR is said to be weak.
In view of the stability, the stability of a boiling water reactor (BWR) includes channel stability, reactor core stability, and region stability. Among them, the channel stability is the thermal hydraulic stability concerning the flow rate changes by the feed back via the changes in pressure drop within a fuel channel (a channel box). The reactor core stability and the region stability mean the nuclear thermal hydraulic stability due to the nuclear feed back via changes in reactivity due to void changes in the reactor core. Furthermore, the reactor core stability is the stability in a basic mode of the neutron flux, in which the output of the entire reactor core integrally changes, while the region stability is stability in a higher mode of the neutron flux in accompany of space changes in reactor core output.
In a conventional BWR, the reactor core consists of a number of fuel assemblies (fuel channels), and on the top and bottom of the reactor core, plenums are provided in common to form a parallel passage system. When the parallel passage system is formed of a number of the fuel channels, even when flow fluctuations are generated in a specific fuel channel, the pressure drop between the plenums on top and bottom of the reactor core is maintained constant due to the presence of the large majority of the other stable fuel channels.
In the reactor core of the parallel passage system, even when flow fluctuations are generated in a specific fuel channel so that the pressure drop is to be changed, a force is applied to the fluid for returning this pressure drop to a predetermined value. The channel stability is stability of a single fuel channel under a boundary condition in that the upper plenum and the lower plenum function as a common pressure boundary of the reactor core so as to maintain the pressure drop of the fuel channel constant.
The fuel channel of the BWR forms a vertical heating passage, and the fluid flowing into the reactor core generates a void due to boiling. The vapor/liquid two-phase flow void-fraction distribution in the reactor core axial direction is like that the void-fraction distribution increases toward the top of the reactor core. Thereby, in accordance with the change in reactor-core inlet flow, the pressure drop of the vapor/liquid two-phase part varies with a time-lag along with the transport lag of the void.
In the vertical heating passage having the vapor/liquid two-phase flow like the reactor core of the BWR, in accordance with the change in inlet flow, the pressure drop of the vapor/liquid two-phase part varies with a time-lag along with the transport lag of the void. This pressure drop of the vapor/liquid two-phase flow with a time-lag becomes a feed back amount of the feed back loop of the channel stability. Generally, with increasing pressure drop through the vapor/liquid two-phase flow, or with increasing time-lag, the channel stability deteriorates.
In the case of the natural-circulation BWR, unlike the reactor core of the BWR, the pressure boundary on the top of the reactor core becomes the outlet of the divided chimney. If the combination of the fuel channel with the divided chimneys is assumed to be an imaginary fuel channel, the region of the vapor/liquid two-phase flow is elongated longer in comparison with the case without the chimneys so that the transport lag of the void is added in the chimneys. Thus, the pressure drop and the time-lag of the vapor/liquid two-phase flow are increased, so that the stability of the imaginary fuel channel may deteriorate.
In the natural-circulation BWR with the divided chimneys, there is no prior art aimed at the improvement in fuel channel stability.
In the natural-circulation BWR with the divided chimney, the multi-dimensional flow is suppressed, so that the flow becomes stable one-dimensional flow to secure the natural-circulation flow rate; however, if the combination of the fuel channel with the divided chimneys is assumed to be an imaginary fuel channel, the region of the vapor/liquid two-phase flow is elongated in the axial direction of the reactor core, so that the transport lag of the void is added in the chimney, which may result in the deterioration in stability of the imaginary channel.
In a conventional BWE, as shown in FIG. 13, the channel stability is evaluated under the condition that the pressure drop Δp in each fuel assembly 1 of the whole reactor core is unified in the upper plenum 2 of the reactor core outlet. In a reactor core 3 of the conventional BWR, several hundreds of the fuel assemblies 1 are arranged, and the nuclear fuel assemblies 1 are loaded in the reactor core 3 to form a parallel passage.
In the reactor core 3 forming the parallel passage, even when fluid vibration is generated in a specific fuel assembly (fuel channel) 1, the vibration is absorbed by a number of fuel channels in its vicinity, so that a feed back effect, in which each channel pressure difference Δp (Δp1 to ΔpN) between the upper plenum 2 and a lower plenum 4 of the reactor core is maintained substantially constant, acts on the channel flow rate.
In the acting process of the feed back effect maintaining the channel pressure difference Δp constant, since the fuel channel is in the vapor/liquid two-phase state, the time-lag from flow rate change to pressure change is generated, so that the fuel channel may be instabilized under a certain vapor/liquid two-phase condition. In a low flow rate and a long passage, in which the time-lag is large due to the change in pressure drop of the vapor/liquid two-phase state, or when the change in pressure drop of the vapor/liquid two-phase flow has large gain, the stability may be more deteriorated.
In the natural-circulation BWR with the divided chimneys, as shown in FIG. 14, pressures of the fuel channels flowing in divided chimneys 6 are once unified (unified in “N” fuel channels 1 with pressure differences ΔpC1 to ΔpCN), and then, the whole fuel assemblies 1 are unified at the outlet of the chimneys 6 (unified in the “k” divided chimneys 6 with pressure differences ΔpCM1 to ΔpCMk).
Thus, if the combination of the fuel assemblies 1 with the divided chimneys 6 is assumed to be an imaginary fuel channel, in the imaginary fuel channel, the region of the vapor/liquid two-phase flow is elongated by the length of the divided chimneys in the axial direction in comparison with the fuel assemblies 1 of the reactor core of the conventional BWR, so that the transport lag of the void is added in the divided chimneys.
Accordingly, in the natural-circulation BWR, the stability, such as the channel stability of the imaginary fuel channel, may be deteriorated.